redients are added in the form of sand and feldspar.
In the formation of slag from these materials the ore is freed
from the silica and aluminium which it contained.

[Illustration: Fig. 85]

~Process.~ The reduction of iron is carried out in large towers called
blast furnaces. The blast furnace (Fig. 85) is usually about 80 ft. high
and 20 ft. in internal diameter at its widest part, narrowing somewhat
both toward the top and toward the bottom. The walls are built of steel
and lined with fire-brick. The base is provided with a number of pipes
T, called tuyers, through which hot air can be forced into the
furnace. The tuyers are supplied from a large pipe S, which circles
the furnace as a girdle. The base has also an opening M, through which
the liquid metal can be drawn off from time to time, and a second
opening P, somewhat above the first, through which the excess of slag
overflows. The top is closed by a movable trap C and C, called the
cone, and through this the materials to be used are introduced. The
gases produced by the combustion of the fuel and the reduction of the
ore, together with the nitrogen of the air forced in through the tuyers,
escape through pipes D, called downcomer pipes, which leave the
furnace near the top. These gases are very hot and contain combustible
substances, principally carbon monoxide; they are therefore utilized as
fuel for the engines and also to heat the blast admitted through the
tuyers. The lower part of the furnace is often furnished with a water
jacket. This consists of a series of pipes W built into the walls,
through which water can be circulated to reduce their temperature.

Charges consisting of coke (or anthracite coal), ore, and flux in proper
proportions are introduced into the furnace at intervals through the
trap top. The coke burns fiercely in the hot-air blast, giving an
intense heat and forming carbon monoxide. The ore, working down in the
furnace as the coke burns, becomes very hot, and by the combined
reducing action of the carbon and carbon monoxide is finally reduced to
metal and collects as a liquid in the bottom of the furnace, the slag
floating on the molten iron. After a considerable amount of the iron has
collected the slag is drawn off through the opening P. The molten iron
is then drawn off into large ladles and taken to the converters for the
manufacture of steel, or it is run out into sand molds, forming the bars
or ingots called "pigs." The